The only one who will know the answer to that is you. You have to figure out the requirements for your particular robot (needed torque, needed precision, needed speed, allowable weight, etc) and then compare the various options that can fulfill those needs.

As a general rule of thumb, I would suggest that, for a walking biped, pneumatics or hydraulics will likely either be too slow, or too imprecise, to be a good choice, although it does likely depend on the weight class of your robot. Are you talking about a two-kilo tabletop bot, or a two-ton war machine prototype?

yes I agree that servo motors are far better than pneumatic actuators in terms of precision and speed.when a bipedal bot walks it should take up 1.3 times the weight.But if my bot weighs around 2 tons,will the servo motor be able to take the load?? OR is there any way to make pneumatic actuators precise by any feedback sensors and will the pneumatic sensors be slow??

Most industrial cranes and other such equipment holding very heavy things, hydaulics is the way to go. You will often see heavy crane arms with significant flex, though, coming both from the inherent flex in the steel in the arm, and the actuators.

Servo motors may very well be able to generate sufficient torque, with an appropriate gearbox. After all, electric dragster cars have a lot of power for a lot of acceleration, and can take the abuse of taking a two-ton vehicle from zero to a lot in a short amount of time. As long as you can properly cool it, the gearbox can take the torque, and you can manage the weight (which is easier when rolling than when walking.) Also, electric trains use electric motors and drivetrains that see a *lot* of abuse!

It's all a question of exactly how much engineering knowledge and experience you have, and how much money you have. To build a two-ton walking robot, you need truckloads of both. (Personally, I'd consider myself to have neither.)